1. Field of the Invention
The present invention relates to a process for producing an actively addressing substrate, and a liquid crystal display using the actively addressing substrate.
2. Description of the Prior Art
A liquid crystal display using a conventional actively addressing substrate has been disclosed in Japanese Patent Application Kokai No. 2-171721, and has a pixel electrode substrate provided with a transparent pixel electrode composed of a large number of ITO films, and a counter substrate provided with a common electrode which is placed face to face with said pixel electrode substrate with a liquid crystal layer placed between the these two substrates.
In the above prior art, in the pixel electrode substrate, gate wiring electrodes formed on a transparent substrate, drain wiring electrodes, the whole of thin film transistors (TFT), and a part of the transparent pixel electrode are covered with a passivation layer made of, for example, silicon nitride (SiN), except for portions not requiring the covering, for example, a connecting terminal portion to an external terminal. On the transparent pixel electrode, said passivation layer is laminated only on the peripheral portion of the electrode and covers said peripheral portion. Furthermore, an alignment layer is laminated on the whole passivation layer and the whole of the portion on which said passivation layer has not been laminated. Thus, the whole transparent pixel electrode substrate is covered with the alignment layer.
In the above prior art, the pixel electrode substrate having such a structure is produced in the following manner. The gate wiring electrode, the drain wiring electrode, the TFT and the transparent pixel electrode are formed on the transparent substrate, and then the silicon nitride layer is formed, for example, by a CVD method. After the coating with the silicon nitride layer photoresist, a photomask is placed thereon and exposure (exposure through the mask) to light is carried out. Then, etching is carried out to remove the silicon nitride layer laminated in the portion of the transparent pixel electrode other than the peripheral portion and the portions not requiring the silicon nitride layer, such as a connecting terminal portion to an external terminal, whereby the passivation layer is formed. Subsequently, the alignment layer is formed on the whole surface on the side on which said passivation layer has been laminated.
In a conventional method, since a photomask is used for patterning, a passivation layer should be left also inside a transparent pixel electrode in view of a margin of error in photomask alignment. Therefore, jogs corresponding to the thickness of the passivation layer are formed inside the transparent pixel electrode. The jogs are disadvantageous as follows. Rubbing treatment of an alignment layer for orienting a liquid crystal uniformly cannot be sufficiently carried out in portions corresponding to the aforesaid jogs, or the pre-tilt angle of liquid crystal molecules does not have a predetermined value in the vicinity of the portions corresponding to the aforesaid jogs. For these reasons and the like, unsatisfactory orientation of the liquid crystal takes place, so that the quality of displayed picture is markedly deteriorated. When the area of a black matrix to be formed on a counter substrate is increased for making the unsatisfactory orientation of said liquid crystal invisible, the aperture ratio is decreased, resulting in the following problems. The picture plane becomes dark, and the amount of electric power consumed is increased for increasing the brightness of a backlight.
Even when a passivation layer is not removed on a transparent pixel electrode, there is a problem that the passivation layer should be removed on the connecting terminal portion to an external circuit in order to improve the electric contact with the external circuit, so that neither the number of masks nor the number of steps can be reduced. In this case, the passivation layer is present on the transparent pixel electrode which applies a voltage to a liquid crystal layer, and hence when a liquid crystal display is operated, the direct-current voltage component of the applied voltage remains in the passivation layer, resulting in phenomena which deteriorate the quality of displayed picture, for example, image-sticking and flicker.
Therefore, Possin et al. carried out exposure to light from behind a substrate in order to remove the disadvantages of error in photomask alignment at the time of exposure, and remove a passivation layer on a gate electrode by etching by a reduced number of steps (U.S. Pat. No. 5,010,027). By this back exposure, a photomask need not be used, so that the problem of error in photomask alignment can be solved, namely, a margin need not be left. When a passivation layer is left as a margin on a gate electrode in a thin film transistor (TFT), a parasitic capacity is present, and hence the performance characteristics of the TFT are deteriorated. In other words, it is conjectured that Possin et al. carried out the back exposure in order to improve characteristics of TFT.